In order to prevent various kinds of products such as food, medicines and metal products, which tend to easily deteriorate due to an influence of oxygen, from oxidizing, oxygen absorbers which remove oxygen from a packaging container or packaging bag have been used. The form of the oxygen absorber which was developed at an earlier stage and is still generally used is a sachet which is filled with a granular or powdery oxygen-absorbing component.
As an oxygen absorbent which can be handled easily, can be widely applied and will not cause problems such as eating it by mistakes, a film- or sheet-shaped (hereinafter sometimes simply referred to as "film") oxygen absorbent exists. This film-shaped oxygen absorbent is used on a packaging container or packaging bag itself. In this case, it is possible to give oxygen-absorbing performance to the packaging container or packaging bag itself.
In order to obtain a film-shaped oxygen absorbent, a technique exists to prepare a resin composition by using a thermoplastic resin as a matrix component and by fixing a granular or powdery oxygen-absorbing component in the matrix component, and to make the resin composition in a multilayered form with other resin layers.
However, the use of this oxygen-absorbing multilayered body sometimes causes the oxygen-absorbing component to contaminate the content of a container when the oxygen-absorbing layer directly contacts a packaged substance such as food, particularly when the content is liquid.
Therefore, as shown in the Japanese Patent (Kokoku) Publication No. SHO 62-1824 and the Japanese Patent (Kokoku) Publication No. SHO 63-2648, a multilayered construction has been devised, in which both sides of the oxygen-absorbing layer are covered with resin layers. For example, the absorbing surface, which is located on the packaged substance side of the oxygen-absorbing layer, is shielded with an air-permeable material and a gas-permeation-resistant layer is placed on the other side of the oxygen-absorbing layer.
If a porous air-permeable material is used for the shielding layer on the absorbing surface of the oxygen-absorbing layer, the oxygen-absorbing component leaks out toward the packaged substance which contains a large amount of moisture. Therefore, it is desirable that the shielding layer is made of a resin layer having no fear of such leakage.
A known oxygen absorber is used as the oxygen-absorbing component to be mixed in the oxygen-absorbing layer. Particularly, iron powder which is superior in the oxygen-absorbing performance is often utilized as a main element for the oxygen-absorbing reaction.
However, the oxygen absorption speed of the conventional oxygen absorbent has remained extremely low. This is because the oxygen permeability of a polyolefine-group resin used as a resin in which the oxygen-absorbing component is mixed is comparatively low, and because the oxygen permeability of the resin shielding layer which covers the absorbing surface of the oxygen-absorbing layer is also low. In other words, the oxygen-absorbing component of the oxygen-absorbing layer is blocked by the resin of the matrix component of the oxygen-absorbing layer and the resin of the shielding layer.
In order to improve the oxygen absorption speed of the oxygen-absorbing layer, as shown in the Japanese Patent Laid-Open (Kokai) Publication No. HEI 2-72851, it is known to form the oxygen-absorbing layer by drawing a resin composition sheet, which is made by mixing and kneading the oxygen-absorbing component containing iron powder as its main element in a thermoplastic resin, so that the resin composition sheet is made microporous. Moreover, the Japanese Patent Laid-Open (Kokai) Publication No. HEI 2-72851 and the Japanese Patent Laid-Open (Kokai) Publication No. HEI 5-162251 describe the technique to draw a multilayered body, in which a shielding layer of a resin composition made by mixing a barely-water-soluble filler in a thermoplastic resin is laminated over a resin composition layer containing an oxygen-absorbing component, so that the resin layer containing the oxygen-absorbing component and the shielding layer are made microporous.
In this case, drawing breaks the interface between the filler and the resin, thereby making micropores. More micropores are connected to each other, causing the entire body to become a continuously porous body. Accordingly, shielding of oxygen permeation by the resin portion decreases and, therefore, the oxygen absorption speed (air permeability) considerably improves. Although it is a porous body, if non-polar or low-polar macromolecules are used, water in a liquid state will not permeate due to the water repellency of such macromolecules.
As described above, The oxygen-absorbing film in the form where the continuously porous oxygen-absorbing layer is covered with the continuously porous shielding layer realizes a high oxygen absorption speed and may be considered as a superior oxygen absorbent which has no problem of contamination when it is used for a short period of time even if the content contains much water.
However, when any liquid having relatively low polarity (for example, when water is not used solely, but is used with an addition of various kinds of fats, oils and alcohol) exists in the content such as food, there is a problem in that the liquid permeates into the micropores of the continuously porous portion and, through the path of such a liquid phase, the oxygen-absorbing component leaks out of the oxygen-absorbing film, causing the contamination of the content.
Also in the case of water, if a gas in the micropores dissipates (for example, dissolves into a liquid) when the oxygen-absorbing body is left for a long period of time, the oxygen-absorbing component sometimes similarly leaks out. It is possible to, for example, use a fluorine-contained agent for treatment in order to give, for example, oil repellency. However, since it will bring about a new fear of contamination by the agent, it is desirable not to use it if possible.
From the viewpoint of preventing the problem of leakage of the oxygen-absorbing component toward the packaged substance, it is desirable that the shielding layer of the oxygen-absorbing layer be a resin layer which is made non-porous. However, if the resin layer becomes thick, the oxygen permeability will not be sufficient. Then, there are known examples in which a thin resin layer is used as the shielding layer. For example, the Japanese Patent Laid-Open (Kokai) Publication No. HEI 5-318675 suggests an oxygen-absorbing multilayered sheet concerning which the oxygen-absorbing resin layer which has been drawn and made microporous is coated with resin.